1. Field of the Invention
This invention is directed to mass flow measuring systems, in general, and to a system wherein a vastly improved operation is achieved with a novel isothermal structural configuration together with a sensing circuit which is speeded up in a novel manner, in particular.
2. Prior Art
For years mass flow meters have been made using a thermal transfer principle, wherein a length of tubing has a pair of heater/sensor coils mounted thereon. Fluid enters the sensor tube at ambient temperature. The fluid conducts heat away from the first coil--(heater/sensor). The heated fluid then continues down the conduct to the second heater/sensor. An additional quantity of heat is again transferred to the fluid. The difference in temperatures of upstream and downstream sensors is an indication of mass flow rate. However, this method of sensing mass flow is relatively slow (a 10 second time constant is typical) and is dependent on the thermal response of the sensor tubing, gas conduction factor, temperature sensor thermal mass and thermal mass of the insulation surrounding the sensor. Electrical compensation has been tried in the sensor circuit but has only provided a small improvement because of the multiple time constants associated with the sensor.
In conventional mass flow measuring systems, a portion of the fluid being detected is passed through a sensing tube. An electrical coil (or dual coil) is wound on the tube. The coil forms a portion of a bridge network. As the resistance of the coil changes, the bridge network detects the flow characteristic. The resistance of the coil changes as a function of its temperature. The coil temperature is affected by the gas flow through the sensing tube. Typically, the bridge network is balanced without fluid flow through the tube and detection of fluid flow is a function of the bridge changes. Reference is herewith made to the copending application entitled IMPROVED MASS FLOW METER WITH REDUCED ATTITUDE SENSITIVITY by J. W. ABOUCHAR and M. J. DOYLE; Serial No. 173,282; filed on July 29, 1980; now U.S. Pat. No. 4,440,021, issued Apr. 3, 1984, to Abouchar et al. The cited copending patent application provides a suitable, detailed description of conventional mass flow measuring systems.
In particular, the referenced patent application describes a dual coil arrangement which is used in measuring the mass flow of a fluid over a wide temperature range with reduced attitude sensitivity. The referenced application specifies the use of an insulating material made of a fibrous material which encases the sensor elements (including the dual coil arrangement) of a measuring system. The apparatus of the co-pending application is a distinct and important improvement over the prior art systems such as those shown and described in U.S. Pat. No. 3,938,384. The prior art system has several distinct disadvantages, as described in the aforementioned application of Abouchar, et al. The Abouchar, et al., application shows and describes an improved system. However, because of the high degree of the sensitivity and tolerance requirements of similar systems, better and improved measuring systems are being sought.
For example, it is desirable to have a method of reducing the effect of the multiple time constants so that the principal time constant becomes dominant. Cancellation of the dominant principal time constant in the electronic control circuitry can then provide a major improvement in speed.